The present invention relates generally to fluorescent lamps operating at very high discharge power density which utilize an evacuable light transmissive envelope coated with phosphor and a gaseous medium within said envelope which can be ionized to excite mercury atoms which emit ultraviolet radiation as well as visible emission of a blue color. In one such type fluorescent lamp, said gaseous medium is ionized by electromagnetic coupling to a source of radio frequency energy, preferably of 50 to 500 kilohertz frequency. A ferrite core located either within or outside the discharge envelope can provide said electromagnetic coupling and the radio frequency energy source may be a solid state oscillator circuit producing a relatively low voltage. A typical lamp of this type includes an evacuable light transmissive envelope coated with phosphor and having a gaseous medium containing mercury vapor within said envelope and which upon excitation by an electric field emits ultraviolet radiation as well as visible radiation of an overall blue color. The structural features of said lamps are described in U.S. Pat. Nos. 4,017,764 and 4,176,296, to Anderson, as well as other U.S. patents referenced therein, all assigned to the present assignee. The operating principles for said lamps are further disclosed in U.S. Pat. Nos. 3,500,118 and 3,521,120, also assigned to the present assignee. Said type lamp is also compact in nature employing a globular shaped envelope with a similar volume as an incandescent lamp and is operated without electrodes, the discharge being induced by the magnetic core at a very high discharge power density. This lamp is adapted to replace incandescent lamps for more efficient generation of white light. As illustrative of the luminous efficacy achieved with one such type prior art electrodeless fluorescent lamp, a 30 watt size lamp is described in the aforementioned U.S. Pat. No. 3,521,120 as demonstrating a luminous efficacy of approximately 40 lumens per watt at an operating temperature of 40.degree. C. with a conventional calcium fluorophosphate phosphor coating which is about three times the luminous efficacy of an incandescent lamp with equal lumen output.
It has also long been recognized that the operating temperature of a conventional tubular type low pressure mercury fluorescent lamp can have a significant effect on luminous efficacy. In said conventional fluorescent lamps, the coldest spot on the lamp wall determines the operating characteristic to a significant degree and is the location where excess mercury condenses. The "cold spot" temperature controls mercury vapor pressure inside the lamp increasing or decreasing the amount of ultraviolet radiation available to excite the phosphor coating. Such conventional fluorescent lamps are generally designed to peak in light output at a cold spot temperature of around 42.degree. C. at which temperature the mercury pressure is about 7 millitorr. Above this value too much mercury vapor is present in the lamp, and some ultraviolet radiation is reabsorbed inefficiently with a subsequent reduction in phosphor excitation per unit of input power. It is further not uncommon when said conventional lamps operate above said optimum cold spot temperature to experience a loss in luminous efficacy of as much as 15-25% and greater. Since the visible mercury vapor radiation escaping through the phosphor coated wall of the lamp envelope in said conventional tubular lamps is usually less than 10% of the total visible emission, however, the white color point of said lamp emission does not change appreciably with variation in the cold spot temperature.
As distinct from the foregoing described operating characteristics of a conventional tubular type fluorescent lamp, as the power density in the mercury discharge is increased, the fraction of the total radiation power from the discharge which is visible radiation also increases. This is understood as being due to a partial saturation of the ultraviolet emission of mercury atoms whereas the visible emission rises in a more nearly linear fashion. As a consequence at very high power densities the visible discharge radiation can amount to 25 to 35% of the total visible emission. Furthermore the efficiency for converting electric power into visible radiation continues to increase well above the aforementioned mercury vapor pressure of 7 millitorr. The overall luminous efficacy for such a lamp therefore attains a maximum value at a significantly higher mercury vapor pressure and cold spot temperature. A further consequence is that the overall lamp luminous efficacy decreases as the power density is increased. Of even more significance to this invention, the overall lamp color point depends significantly on both the power density of the discharge and the temperature of the cold spot.
It is also known to utilize phosphor combinations of various kinds in the conventional fluorescent lamp construction of a tubular type wherein either blended mixtures of the individual phosphor constituents or even multiple layers of the individual phosphor constituents, including mixtures thereof, are utilized. For example, there is described in U.S. Pat. No. 4,075,532 to Piper et al, also assigned to the present assignee, a phosphor blend utilizing a first phosphor having a relatively narrow emission band peaking in the short visible wavelength (blue) region and a second phosphor having the relatively broad band emission peaking in the 570-600 nanometer (yellow) region of the visible spectrum which provides improved luminous efficacy in this type fluorescent lamp construction. As further illustrative of a different phosphor combination producing white light more efficiently than conventional deluxe type fluorescent lamps of a tubular type construction, there is described in U.S. Pat. No. 4,079,287 issued to Soules et al and assigned to the present assignee, the phosphor blend utilizing a strontium haloapatite phosphor and a europium-activated yttrium oxide phosphor. A still different phosphor combination said to produce warm white color light efficiently in said conventional low pressure fluorescent lamp is described in U.S. Pat. No. 4,088,923 as a blended mixture of two magnesium aluminate phosphors with a hexagonal crystal structure and activated with specific rare earth ions and a third phosphor of yttrium oxide activated with trivalent europium.
The warm white color generally sought in these lamps for a direct replacement of incandescent lamps at a far greater luminous efficacy cannot be achieved with conventional halophosphates, such as calcium haloapatite phosphor, or even with more recently developed phosphor combinations utilizing various halophosphate phosphor components above a certain level of discharge power density. Primarily, said phosphor materials lack color points which can be adjusted to compensate for the significant visible mercury vapor radiation being emitted from said higher power density fluorescent lamps to produce lamp emission of a warm white color. If the cold spot temperature is reduced below the aforementioned operating temperatures to produce a white color point for the lamp emission closer to a warm white color, there is experienced an unacceptable loss in luminous efficacy. There is further experienced a much greater lumen depreciation during a lifetime in said lamps than occurs in the tubular fluorescent lamps using the same conventional halophosphate phosphors and which becomes more pronounced at high discharge power density of lamp operation. Accordingly, an improved phosphor is desired permitting a high power density fluorescent lamp such as the electrodeless lamp to operate with acceptable lumen efficacy during its lifetime and which can also produce various white color points for the lamp emission by adjustment of the operating cold spot temperature.